Application of multilayered piezoelectric ceramic devices such as piezoelectric ceramic transformer and piezoelectric actuators is increasing recently. However, multilayered piezoelectric ceramic devices made of conventional lead zirconate titanate (PZT) materials have to be cofired with Pt, Pd or Ag—Pd paste due to its high sintering-temperature. High cost of these internal electrodes limits widespread use of multilayered piezoelectric ceramic devices. Therefore, there were extensive efforts to develop low-temperature firable PZT materials, preferably below the melting point of pure silver (Ag), 962° C. In order to achieve a high efficiency under dynamic operations such as in piezoelectric ceramic transformer, low-loss hard piezoelectric ceramic materials are required with a high piezo modulus dij, a high electro-mechanical coupling coefficient, kp, and a high dielectric constants. But, there were many difficulties in lowering sintering-temperature without deteriorating piezoelectric properties of hard piezoelectric ceramic. Sintering temperature of PZT materials can be lowered simply by adding glass frits with low melting points to accelerate densification at a low temperature by liquid phase sintering but it often degrades piezoelectric parameters. References are made to U.S. Pat. No. 5,792,379; Gui et. al., J. Am Ceram Soc. 72[3], 486-91(1989); U.S. Pat. No. 5,433,917; S. Y. Cheng et. al., J. Mat. Sci., 21, 571-576(1986); D. E. Witmer and R. C. Buchana, J. Am Ceram. Soc., 64, 485-490(1981).
U.S. Pat. No. 5,792,379 teaches PZT ceramic composition having a reduced sintering temperature down to 900° C., enabling cofiring with pure silver paste as an internal electrode material. Sintering temperature was lowered using sintering aid made of B2O3, Bi2O3, MeO and CuO, where Me is one of Ca, Sr, Ba and Zn. However, tradeoff with a lower sintering temperature achieved was a reduction of kp, at best 0.55 compared to that of hard piezoelectric ceramics with kp over 0.60.
U.S. Pat. No. 5,433,917 teaches PZT compositions with a low sintering temperature of about 1,000° C. using eutectic mixtures of CuO and oxides of alkaline earth metals but with no detailed piezoelectric parameters reported except dielectric constants and dielectric losses. Gui et. al. also described a method to reduce sintering temperature of PZT using a small amount of B2O3, Bi2O3 and CdO. But sintering temperatures for both prior arts are still too high to cofire with pure Ag electrode.
In these prior arts, glass frits with a low melting point are used as a sintering aid but all the elements in glass frits are not usually incorporated into perovskite strucuture of PZT matrix in sintering, for example boron. They retained in glassy phase along the grain boundaries after sintering and thus deteriorate piezoelectric properties.